10:45 AM - 12:15 PM
[BCG06-P03] Formation processes of calcium carbonate precipitates covering water surface and gas bubbles
Travertine is a hot spring carbonate deposit formed by CaCO3 precipitation resulting from CO2 degassing from hot spring water. While CaCO3 precipitation often occurs at the solid-liquid interface such as the bottom of the water, the gas-liquid interface such as the water surface and bubbles are sometimes covered with CaCO3; they are called as paper-thin rafts and coated bubbles, respectively. Previous studies interpreted that they are formed by CO2 degassing at the gas-liquid interface and subsequent in-situ precipitation of CaCO3, but its validity is still unclear. The purpose of this study is, therefore, to clarify the origin of paper-thin rafts and coated bubbles observed in Nagayu hot spring (Oita Pref.), by observing the air-liquid interface in a microscopic scale.
First, conventional thin sections were prepared by embedding both samples in resin, and observations were made using a polarizing microscope and a confocal laser scanning microscope. Next, a thin-foil sections were prepared from the air-liquid interface of the conventional thin sections by focused ion beam processing, and observed using a transmission electron microscope and a scanning transmission X-ray microscope.
The microscopic observations indicated that aragonite crystals were densely packed at the air-liquid interface in the paper-thin raft. On the other hand, in the coated bubble, acicular aragonite crystals were loosely arranged at the gas-liquid interface, and many voids were also observed. These observations suggest that the paper-thin raft is formed by the in situ precipitation of fine aragonite, while the coated bubble is formed by the attachment of acicular aragonite formed elsewhere to the surface of the bubble. The lack of in situ precipitation on the bubble surface is considered to be due to the lack of continuous CO2 degassing, because the CO2 partial pressure of bubbles with a small volume quickly equilibrates with the hot spring water.
First, conventional thin sections were prepared by embedding both samples in resin, and observations were made using a polarizing microscope and a confocal laser scanning microscope. Next, a thin-foil sections were prepared from the air-liquid interface of the conventional thin sections by focused ion beam processing, and observed using a transmission electron microscope and a scanning transmission X-ray microscope.
The microscopic observations indicated that aragonite crystals were densely packed at the air-liquid interface in the paper-thin raft. On the other hand, in the coated bubble, acicular aragonite crystals were loosely arranged at the gas-liquid interface, and many voids were also observed. These observations suggest that the paper-thin raft is formed by the in situ precipitation of fine aragonite, while the coated bubble is formed by the attachment of acicular aragonite formed elsewhere to the surface of the bubble. The lack of in situ precipitation on the bubble surface is considered to be due to the lack of continuous CO2 degassing, because the CO2 partial pressure of bubbles with a small volume quickly equilibrates with the hot spring water.